Accurate bandstructure calculations are important for the design and analysis of photonic crystal devices. In this example, we will focus on how to use MODE Solutions 2.5D FDTD propagation method to calculate the bandstructure of a slab photonic crystals device with a square and hexagonal lattice of holes.-Accurate bandstructure calculations are important for the design and analysis of photonic crystal devices. In this example, we will focus on how to use MODE Solutions'　2.5D FDTD propagation method to calculate the bandstructure of a slab photonic crystals device with a square and hexagonal lattice of holes.